A reduced row echelon form of a 3 x 4 augmented matrix can have how many leading 1's?

a) must have 3
b) may have 1, 2, or 3
c) may have 0, 1, 2, or 3
d) may have 0, 1, 2, 3, or 4
e) must have 4

Does the answer change for a 2 x 3 augmented matrix?

For a 15 x 16 augmented matrix?

For a 2 x 4 augmented matrix?